1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copier and others.
2. Description of the Related Art
Heretofore, there are various image forming apparatuses adopting an electro-photographic or electrostatic recording image forming process, and as one of such various image forming apparatuses, there is a tandem-type image forming apparatus including process cartridges arrayed in a row removably within a body of the image forming apparatus and configured to form a color image.
In such a tandem-type image forming apparatus, e.g., a contact developing type image forming apparatus, there is a possibility of causing the following problems if a developing state in which a developing bias is applied to a developing unit is continued until a non-image forming time during which no exposure is executed by an exposure unit:    (1) Scraping of a surface layer of a photosensitive body (photoconductive drum or the like) caused by being rubbed by a developing roller;    (2) Shortening of a life of photosensitive bodies of other colors not involved in development during monochrome printing in a black (Bk) monochrome mode;    (3) Waste of developer and adhesion of the developer to a recording medium during a non-developing operation during which no developing bias is applied; and    (4) Deformation of a developing roller caused by being not used for a long period of time.
Meanwhile, there is a possibility that the following problems occur in the same manner also in a non-contact developing type image forming apparatus if the developing state in which a developing bias is applied to the developing unit is continued until when the non-image forming time during which no exposure is executed by the exposure unit;    (5) Deterioration of developer and toner; and    (6) Increase of consumption of toner.
In order to solve the above mentioned problems, Japanese Patent Application Laid-open No. 2006-323235 proposes an image forming apparatus configured to operate such that the developing unit is put into the developing state only when an image of own color is to be formed for example. This apparatus is configured such that the developing units corresponding to yellow (Y), magenta (M), cyan (C) and black (Bk) are put into the developing states in this order in response to a start of an image forming operation of own color. Then, as soon as the image forming operation of own color ends, the developing units corresponding to the yellow (Y), magenta (M), cyan (C), and black (Bk) toner images are put into the non-developing states in this order.
However, according to the configuration described in Japanese Patent Application Laid-open No. 2006-323235, the following problem may occur if a coefficient of friction μl of a driving roller driving while supporting an intermediate transfer belt drops due to adhesion of flying toner and paper powder while repeating the image forming operations (durability). That is, there is a case where a slip occurs between the intermediate transfer belt and the driving roller, causing a minute fluctuation of speed of the intermediate transfer belt, during when the developing units are switched to the developing states.
One reason of this problem is that a coefficient of friction μ2 between the photoconductive drum and the intermediate transfer belt when toner exists in the primary transfer nip portion between the primary transfer roller and the intermediate transfer belt is different from a coefficient of friction μ3 between the photoconductive drum and the intermediate transfer belt when no toner exists in the photoconductive drum.
That is, assuming nip pressure between the driving roller and the secondary transfer roller facing with each other and pinching the intermediate transfer belt as F1, nip pressure of the primary transfer nip portion as F2, and that there exist four photoconductive drums, their relationship in an initial state is differentiated after forming a certain number of images. That is, the relationship in the initial state of:μ2×F2×4<μ3×F2×4<μ1×F1is changed to a relationship of:μ2×F2×4<μ1×F1<μ3×F2×4after forming the certain number of images.
This tendency is more remarkable in a small type image forming apparatus in which a diameter of a driving roller of an intermediate transfer belt is small.
In the case where the minute fluctuation of speed of the intermediate transfer belt occurs as described above, there is a problem that color shift correction becomes insufficient due to a minute detection error also in a color shift correction control conducted by forming a color shift correcting pattern image on the intermediate transfer belt as follows. That is, the problem occurs in a so-called automatic registration adjustment of adjusting timings for exposing the photoconductive drums from a scan type optical unit per each color by detecting the correcting pattern image on the intermediate transfer belt by a color shift amount detecting portion such as a CCD sensor and by detecting a color shift amount of each color.
After forming the certain number of images, if the developing units are put into the developing state only in forming own image as described in the configuration of Japanese Patent Application Laid-open No. 2006-323235, the other developing units of magenta, cyan and black are put into the non-developing state in forming an initial yellow image, and the state of coefficient of friction μ3 in which there exists no toner in the primary transfer nip portions of magenta, cyan and black is brought about. Due to that, a slip tends to occur between the intermediate transfer belt and the driving roller even though it is a level in which unevenness is unnoticeable as an image shock. If the slip occurs between the intermediate transfer belt and the driving roller during the so-called auto-registration, intervals of each color of the color shift correcting pattern formed on the intermediate transfer belt become unstable, i.e., the color shift correction becomes insufficient, possibly causing a color shift during printing on a sheet.